The adverb 'thickly' is formed by adding the productive adverbial suffix '-ly' to the adjective 'thick'. The core adjective 'thick' originates from the Old English þicce, which carried meanings related to density, viscosity, and closeness. This Old English term is traced back to a Proto-Germanic root þekuz, itself derived from the Proto-Indo-European root tegu-, meaning 'thick' or 'fat'. The addition of '-ly', a suffix with ancient Germanic origins used to form adverbs from adjectives, straightforwardly transforms the quality of being dense or closely packed into the manner in which an action is performed, resulting in the modern sense of 'in a thick manner'.

Structure: thick (dense) + ly (adverb suffix) 'thick' is from Old English þicce (dense, viscous, numerous), of Germanic origin. 'ly' is an Old English adverbial suffix, from Proto-Germanic -līko.