1530s, "promptly," from mature (adj.) + -ly (2). Sense of "with full deliberation" is from 1590s; that of "in a way indicative of maturity" is from 1841.
she acts very maturely for her age
mattock
mattress
maturate
maturation
mature
maturely
maturescent
maturity
matutinal
matzah
matzoh
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