Plainly

wiktionary

ref

From Middle English playnly, pleinly, pleyneliche, equivalent to plain +‎ -ly.

etymonline

ref

plainly (adv.)

late 14c., "frankly, candidly; without a doubt, truly; in a clear and distinct manner; simply, straightforwardly," from plain (adj.) + -ly (2). Meaning "simply, frugally" is from 1560s.