Partly means in some measure, though not wholly. Partly true, partly because of the weather, partly to blame.
It is the honest word for a mixed answer, and partly true is often the most accurate thing that can be said. Partly and partially overlap; the useful distinction is that partly divides a whole into parts and partially means incompletely.
1adverbpartlyIn some measure, though not wholly.
“The accident was partly my fault, because I wasn't looking.”
partly is a degree.
Proto-Indo-European
*perh₃-
Proto-Indo-European
*par-
Latin
partem
Old French
part — part
Old English
part — part
Middle English
part
English
part
English, today
partly
Built from
From part + -ly. part comes from Middle English part, from Old English part (“part”) and Old French part (“part”); both from Latin partem, accusative of pars (“piece, portion, share, side, party, faction, role, character, lot, fate, task, lesson, part, member”), from Proto-Indo-European par-, per- (“to sell, exchange”). Akin to Latin portiō (“a portion, part”), parō (“to make ready, prepare”). del, dele (“part”) (from Old English dǣl (“part, distribution”) > Modern English deal (“portion; amount”)), Middle English dale, dole (“part, portion”) (from Old English dāl (“portion”) > Modern English dole), Middle English sliver (“part, portion”) (from Middle English sliven (“to cut, cleave”), from Old English tōslifan (“to split”)).
Etymology from Wiktionary, CC BY-SA