Sapskull is a compound of sap ('soft sapwood', hence figuratively 'soft-headed') + skull, on the model of numbskull; sap is from Old English sæp. The historical chain includes Old English sæp. Older glosses in the chain include "soft sapwood" and "soft-headed", which help explain the semantic range of the modern word. Morphologically, sapskull is best read as compound and borrowing, yielding an English noun shaped by both form history and later usage. In current English it functions as a noun, with the sense "a stupid or dull-witted person; a blockhead". related forms include sap, saphead, and sappy. The result is an English noun whose etymology preserves the older forms while supporting the word's specialized modern meaning.