An Etymological Dictionary of Astronomy and Astrophysics

1a) Deficiency or absence of something needed, desirable, or customary.

1b) Something missing or needed. See also → default, → deficiency, → shortage.

2a) (v.tr.) To be without or deficient in; to fall short in respect of.

2b) (v.intr.) to be absent or missing, as something needed or desirable (Dictionary.com).

Etymology (EN): M.E. lak; cognate with M.L.G. lak, M.Du. lac “deficiency;” akin to O.Norse lakr “deficient.”

Etymology (PE): Nast, from negation prefix na- “in-, non,” → not, + ast, hast “is,” from astan, hastan “to be,” → exist.