“brave day sunk in hideous night”
'Brave' here means splendid or magnificent — not courageous. The pairing of 'brave' (brilliant, fine) against 'hideous' drives home the contrast: a glorious day is swallowed by ugly darkness. The sense of 'brave' as handsome or grand was common in Elizabethan English and survives in phrases like 'a brave show.'
“sable curls, all silvered o'er with white”
'Sable' means black — borrowed from heraldry, where it names the black tincture on a coat of arms. The image is of dark hair turning white with age: black (sable) curls gradually covered in silver. The heraldic word lends the moment a formal, almost ceremonial weight, as if recording the extinction of something noble.
“erst from heat did canopy the herd”
'Erst' means 'once' or 'formerly' — it is a now-obsolete adverb common in Elizabethan poetry. 'Canopy' is used as a verb: the trees once served as a canopy (an overhead shade or cover) for livestock sheltering from summer heat. The line pictures the same trees that now stand bare leaves as they were in summer.
“summer's green all girded up in sheaves, / Borne on the bier with white and bristly beard”
A 'bier' is the frame or carriage used to carry a coffin at a funeral. Here the poet makes the grain harvest into a funeral procession: summer's green stalks, bound into sheaves and crowned with white ears, are carried to burial like a bearded corpse. The image turns the year's abundance into a body being laid to rest, making the harvest — normally a sign of plenty — a sign of death.
“nothing 'gainst Time's scythe can make defence / Save breed, to brave him when he takes thee hence”
'Time's scythe' refers to the ancient image of Father Time or the Grim Reaper carrying a curved blade to cut down life the way a farmer cuts grain. 'Breed' means offspring — only children can defy this reaper. 'Brave' here is a verb meaning 'to defy' or 'to challenge,' completing a wordplay across the sonnet: 'brave' began as an adjective (line 2, splendid day) and returns as a verb (brave him, face him down). The couplet's entire argument is: nothing defeats Time except children, who carry on your beauty after you are gone.
historical The procreation argument (Sonnets 1-17)