Lewis & Short

ămussis, is, f. [etym. unc.; perh. from am- and assis = axis, a plank, i. e. something flat, straight, moved about a surface in adjusting it] (acc. amussim, v. Neue, Formenl. I. p. 198; abl. and plur. not used; only ante- and post-class.), a rule or level, used by carpenters, masons, etc.: amussis: tabula, quā utuntur ad saxa leviganda, Varr. ap. Non. p. 9, 17; Aus. Idyll. 16, 11; cf. Sisenn. ap. Charis. p. 178 P.; Paul. ex Fest. p. 6 Müll.
In class. Lat. in the adv. phrases,

  1. I. ad ămussim (also written as one word, ad-ămussim or ătamussim), according to a rule or level, i. e. accurately, exactly: adamussim non est numerus, Varr. R. R. 2, 1, 26: talionem ad amussim aequiparare, Gell. 20, 1, 34 Hertz: ut judicium esse factum atamussim diceres, id. 1, 4, 1 id.
  2. II. exămussim, according to a rule, exactly, quite: Ne ista edepol, si vera haec loquitur, examussimst optuma, Plaut. Am. 2, 2, 213 (with the forms adamussim and examussim, cf. the Gr. ἐκποδών and ἐμποδών).