Used to introduce the first point in a series of things being discussed or listed.
From 'first' (Old English 'fyrst') plus the adverbial suffix '-ly' (from Old English 'lic' meaning 'like' or 'body'). The suffix '-ly' turns adjectives into adverbs.
English speakers actually don't need 'firstly'—'first' works as an adverb by itself, and this bothers grammar purists. Yet 'firstly' became popular in formal speech because it sounds more authoritative, even though it's technically redundant.
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