relapse (v.)
early 15c., "renounce;" 1560s, "fall into a former state," from Latin relapsus, past participle of relabi "slip back, slide back, sink back," from re- "back" (see re-) + labi "to slip" (see lapse (n.)). Related: Relapsed; relapsing.
relapse (n.)
mid-15c., from relapse (v.).