An archaic past participle meaning sung beforehand or previously uttered in song; sung excessively.
From Old English 'for-' + past participle of 'sing.' Following the pattern of Germanic verb formation, this creates a past participle showing an action completed with emphasis or prior to another action.
This dead word shows the ghost of how Old English worked—it probably appeared mainly in poetry, where rhyme and meter would demand 'forsung' over 'sung previously' to fit the line.
Complete word intelligence in one call. Free tier — 50 lookups/day.