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Why it's 'die Uni' but 'der Akku': short forms keep the gender of the full word

German loves short forms. Students go to the Uni, your phone's Akku runs flat, and you put on some Deo in the morning. These words are cut down from longer ones (Universität, Akkumulator, Deodorant), and they almost always keep the gender of that longer word: die Uni, der Akku, das Deo.

This catches English speakers out because the short forms end in sounds that seem to suggest a gender. If you know some Spanish or Italian, a final -o feels masculine and a final -i feels like nothing in particular. German ignores that. The ending of the short form tells you very little. What counts is the word it came from.

Word list
AutonA1 car das Auto ← das Automobil BusmA1 bus der Bus ← der Omnibus KilonA1 kilogram das Kilo ← das Kilogramm PCmA1 computer der PC ← der Personal Computer UnifA1 university die Uni ← die Universität ZoomA1 zoo der Zoo ← der zoologische Garten KlonA2 toilet das Klo ← das Klosett LKWmA2 truck der LKW ← der Lastkraftwagen MathefA2 mathematics die Mathe ← die Mathematik MayofA2 mayonnaise die Mayo ← die Mayonnaise TramfA2 tram die Tram ← die Trambahn (in Switzerland: das Tram) BHmB1 bra der BH ← der Büstenhalter LabornB1 laboratory das Labor ← das Laboratorium SoapfB1 soap opera die Soap ← die Soap-Opera AkkumB2 battery der Akku ← der Akkumulator DeonB2 deodorant das Deo ← das Deodorant TachomB2 speedometer der Tacho ← der Tachometer UFOnC1 UFO das UFO ← das unbekannte Flugobjekt
How it works

There are two main ways German shortens a word. Either it simply chops off the end (Mathematik becomes Mathe, Kilogramm becomes Kilo), or it turns the word into letters (Lastkraftwagen becomes LKW, Büstenhalter becomes BH). In both cases the article stays the same as before.

English has no grammatical gender, so this never comes up in English. In German it decides the article, the adjective endings and words like ein, mein or ihr. The rule of thumb is simple: if you know the long word, you know the gender of the short one. If you only know the short form, learn it with its article from the start.

The words
Feminine: die

Uni comes from die Universität, Mathe from die Mathematik and Mayo from die Mayonnaise. Soap is short for die Soap-Opera, and the German word for it, die Seifenoper, is feminine too. Note that Mathe is often used without an article, as in Ich habe Mathe.

An der Uni ist die Mathe viel schwerer als in der Schule.
(At university, the maths is much harder than at school.)
Kannst du mir bitte die Mayo geben?
(Can you pass me the mayo, please?)
Meine Oma schaut jeden Abend ihre Soap.
(My grandma watches her soap every evening.)
Masculine: der

Akku comes from der Akkumulator and Tacho from der Tachometer. The letter words LKW (der Lastkraftwagen), BH (der Büstenhalter) and PC (der Personal Computer) follow the same logic. Zoo is short for der zoologische Garten, so it takes the gender of Garten.

Am E-Bike ist der Akku leer, und der Tacho geht auch nicht mehr.
(The battery on the e-bike is dead, and the speedometer has stopped working too.)
Der LKW blockiert die ganze Straße.
(The lorry is blocking the whole street.)
Sie kauft sich einen neuen BH.
(She is buying herself a new bra.)
Ich brauche einen neuen PC für die Arbeit.
(I need a new computer for work.)
Am Sonntag gehen wir in den Zoo.
(On Sunday we're going to the zoo.)
Neuter: das

Auto comes from das Automobil, Kilo from das Kilogramm, Klo from das Klosett, Deo from das Deodorant and Labor from das Laboratorium. UFO stands for das unbekannte Flugobjekt, so it is neuter like Objekt.

Das Auto steht in der Garage.
(The car is in the garage.)
Das Kilo Tomaten kostet drei Euro.
(A kilo of tomatoes costs three euros.)
Entschuldigung, wo ist hier das Klo?
(Excuse me, where's the toilet here?)
Das Deo riecht nach Zitrone.
(The deodorant smells of lemon.)
Das Labor ist im dritten Stock.
(The laboratory is on the third floor.)
Hast du schon mal ein UFO gesehen?
(Have you ever seen a UFO?)
Watch out

This is a very strong tendency, but it is not an exceptionless rule. The best-known exception is das Foto. It is neuter, even though die Fotografie is feminine. Learn it as a special case.

Many other everyday short forms behave as expected: die Kita (die Kindertagesstätte), das Abi (das Abitur), die Kripo (die Kriminalpolizei) and die Lok (die Lokomotive).

Do not confuse true short forms with shortened compounds such as die U-Bahn (Untergrundbahn) or das U-Boot (Unterseeboot). There the last part, Bahn or Boot, is still a complete word, and it gives the gender in the normal way.

The typical mistake is guessing from the sound: saying der Auto or der Deo because of the -o, or das Uni because -i feels neutral. When in doubt, think of the long word.

Summary

A clipped word or an abbreviation keeps the gender of the full word it comes from, whatever its new ending sounds like: die Uni because of die Universität, der Akku because of der Akkumulator, das Deo because of das Deodorant. Ignore the final -o or -i, think of the long form, and remember the one famous exception, das Foto.