c. 1300, "dispiritedly, timidly, half-heartedly;" early 14c. "feebly, wearily, without vigor;" from faint (adj.) + -ly (2). Meaning "indistinctly" is from 1580s. Also in Middle English, "deceitfully, hypocritically, falsely" (mid-14c.).
stars shining faintly through the overcast
the rumors weren't even faintly true
could hear his distant shouts only faintly
between him and the dim light a form was outlined faintly