A side staller is by definition:

side·staller ( P ) Pronunciation Key (sid*stllr)
adjective / noun / verb

A side staller is a person that enters a public bathroom and enters the stall directly next to another individual. Side stallers are usually the lowest class, filthy people in a corporate environment. Habitual side stallers are frequently targets of bathroom beatings and practical jokes.

Also can be used as a verb, as in "I got sidestalled."
"No, a sidestaller?" adjective.

A portmanteau of carpe diem, a Latin phrase that means “Seize the Day!” and DM, which stands for Direct Messages. This word means seizing the day via DMs.