Lewis & Short

sub-lābor, lapsus, 3, v. dep. n., to fall, glide, or sink under or down, to sink (poet. and in post-Aug. prose).

  1. I. Lit.: imperfecta necesse est labent et modo prodeant, modo sublabantur aut succidant, Sen. Ep. 71, § 34: aedificia vetustate sublapsa, Plin. Ep. 10, 75, 1; cf. poet. transf.: annis sublapsa vetustas, Verg. A. 12, 686.
    1. * B. In partic., to slip down, glide away: lues udo sublapsa veneno Pertentat sensus, Verg. A. 7, 354.
  2. II. Trop.: retro sublapsa Spes, Verg. A. 2, 169: memoria senum, Sen. Oedip. 817.