Where does “billygoatweed” come from?

billygoatweed (English) comes from English weed, from Middle English weed, from Old English wēod, from Proto-West Germanic *weud — weed (unwanted plant).

Ancestry of “billygoatweed”, step by step

billygoatweed traces back through two separate lines of ancestry.

via English weed

StepLanguageWordMeaning
1EnglishweedAny plant regarded as unwanted at the place...
2Middle Englishweed
3Old Englishwēodweed
4Proto-West Germanic*weudweed (unwanted plant)

via English billygoat

StepLanguageWordMeaning
1Englishbillygoat
Every word from Proto-West Germanic *weudEvery word from Old English wēodEvery word from Middle English weed