Rep was short for reputation: I’ll stake my rep on it.

That’s pos or pozz (for positive).

A fickled crowd is a mob, from the latin mobile vulgus.

Clippings are very common in the history of English: ad, celeb, prof, doc, phone, burger, flu, fridge, fax, memo, gym, exam, vet, pub, bus (from omnibus), cello (from violoncello)…