The Collaborative International Dictionary of English v.0.48:

Illapse \Il*lapse"\, v. i. [imp. & p. p. Illapsed; p. pr. &
   vb. n. Illapsing.] [L. illapsus, p. p. of illabi; pref. il-
   in + labi to fall, slide.]
   To fall or glide; to pass; -- usually followed by into.
   --Cheyne.
   [1913 Webster]