Why is the ninth month called September?
prefix of negation, Old English un-, from Proto-Germanic *un- (cf. Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (cf. Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), a variant of PIE root *ne- "not" (cf. Avestan na, Old Church Slavonic and Lithuanian ne "not," Latin ne "that not," Greek ne- "not," Old Irish ni, Cornish ny "not").
Freely and widely used since Old English in compounds with native and imported words, it disputes with Latin-derived cognate in- the right to form the negation of certain words (indigestable/undigestable, etc.). Often euphemistic (e.g. untruth for "lie"). The most prolific of English prefixes, it even is used to make words from phrases (e.g. uncalled-for, c.1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but not restricted to un-; cf. put-up-able-with, 1812). As a prefix in telegram-ese to replace not and save the cost of a word, it is first attested 1936.
prefix of reversal (e.g. unhand, undo, unbutton), Old English on-, un-, from Proto-Germanic *andi- (cf. Old Saxon ant-, Old Norse and-, Dutch ont-, Old High German ant-, German ant-, Gothic and- "against"), from PIE *anti "facing opposite, near, in front of, before" (see ante).