Etymology 1

• Learned borrowing from Latin cum (“with”).

Preposition

1. Used in indicating a thing or person which has two or more roles, functions, or natures, or a which has changed from one to another.

Etymology 3

Adjective

1. Clipping of cumulative.

Etymology 4

Noun

Abbreviation of cubic metre. (ex: The density of cement is 1440 kg/cum.)

For further definitions omitted see: https://en.wiktionary.org/wiki/cum

Why this one? Because I used it in a joke and some apparently did not know the primary use as a preposition.