teeth

For no particularly good reason, one has a toothache, not a teethache, even if more than one tooth is hurting. One also refers to a toothbrush and to tooth marks, although the brush works on more than one tooth and marks result from the bite of teeth. Teeth, the plural of tooth, outscores the singular form in the number of hackneyed expressions in which both appear: "long in the tooth" ("old," "elderly"),"tooth and nail" ("fiercely," "as hard as possible"),"a toothsome invitation," "by the skin of one's teeth," "a kick in the teeth," "put teeth in (or into)," "show one's teeth," "put (or set) one's teeth on edge," "to the teeth" ("entirely," "fully"),"to throw into someone's teeth" ("to reproach"), and "cut one's teeth on" (referring to action during one's youth).