verb[with object] archaic
Old English brēc (plural of brōc, of Germanic origin; related to Dutch broek), interpreted as a singular form. The original sense was 'garment covering the loins and thighs' (compare with breeches), hence 'the buttocks' (breech (sense 2 of the noun), mid 16th century), later 'the hind part' of anything
Do not confuse breach with breech. Breach means 'break an agreement or rule' (they breached the Data Protection Act), 'break through something', or 'a gap', whereas breech means 'the back part of a gun barrel'.