toothbrush (n.)

Old English toð (plural teð), from Proto-Germanic *tanthu- (source also of Old Saxon, Danish, Swedish, Dutch tand, Old Norse tönn, Old Frisian toth, Old High German zand, German Zahn, Gothic tunþus), from PIE root *dent- "tooth." Plural teeth is an instance of i-mutation.

The loss of -n- before spirants is regular in Old English, Old Frisian, and Old Saxon: compare goose (n.), five, mouth (n.). Also thought, from stem of think; couth from the stem of can (v.1); us from *uns.

Application to tooth-like parts of other objects (saws, combs, etc.) first recorded 1520s. Tooth and nail as weapons is from 1530s. The tooth-fairy is attested from 1964.

"instrument consisting of flexible material (bristles, hair, etc.) attached to a handle or stock," late 14c., "dust-sweeper, a brush for sweeping," from Old French broisse, broce "a brush" (13c., Modern French brosse), perhaps from Vulgar Latin *bruscia "a bunch of new shoots" (used to sweep away dust), perhaps from Proto-Germanic *bruskaz "underbrush." Compare brush (n.2). As an instrument for applying paint, late 15c.; as an instrument for playing drums, 1927. Meaning "an application of a brush" is from 1822.