limbus (n.)

late 14c., "edge of a quadrant or other instrument," from Latin limbus "ornamental border, hem, fringe, edge," a word of uncertain origin. Klein suggests it is cognate with Sanskrit lambate "hang down limply" and English limp (adj.). Tucker writes that "the sense appears to be that of something which twists, goes round, or binds ... not of something which hangs loose," and suggests cognates in Lithuanian linta "ribbon," Old Norse linnr "whether." De Vaan tends to agree with Klein and writes, "In view of the phoneme *b, the very specific meaning of limbus and its absence from the oldest literature, the etymology remains uncertain." Astronomical sense of "edge of the disk of a heavenly body" first attested 1670s. Related: Limbal.

region supposed to exist on the border of Hell, reserved for pre-Christian saints (Limbus patrum) and unbaptized infants (Limbus infantum);" c. 1300, from Latin limbo, ablative singular of limbus "edge, border" (see limb (n.2)). In frequent use in Latin phrases such as in limbo (patrum), which is entirely Latin, but the in was taken as English and hence the Latin ablative became the English noun. Figurative sense of "condition of neglect or oblivion, place of confinement" is from 1640s.