papillon

butterfly; someone brilliant, versatile and inconstant; knot; ellipsis of écrou papillon (“wing nut, butterfly nut”); butterfly stroke

“vif et léger comme un papillon” — lively and light like a butterfly

From Old French papeillon (12th c.), byform of paveillon, whence the modern doublet pavillon (“pavilion”). Both from Latin pāpiliōnem. The form with recovered -p- may be semi-learned, but has also been explained as a native development due to onomatopoeia. Another possibility would be influence by a hypothetical Old French *parpeillon, from Vulgar Latin *parpiliōnem (cf. Occitan parpalhon). The swimming sense is a semantic loan from English.