Subtly means in a way so slight it is easy to miss. Subtly different, subtly changed, put it subtly.
It is one of the most commonly misspelled adverbs in English, since the b is silent and the e of the adjective disappears. Subtly different is the standard phrase for a difference that matters more than it looks.
1adverbsubtlyIn a way so slight it is easy to miss.
“She subtly changed the subject when he asked about her salary.”
subtly is a manner.
From subtle plus -ly, from Latin subtilis, finely woven.
Middle English
sotilly
English, today
subtly
From Middle English sotilly; equivalent to subtle + -ly.
Etymology from Wiktionary, CC BY-SA