Old English ebba "falling of the tide, low tide," perhaps from Proto-Germanic *af- (source also of Old Frisian ebba, Old Saxon ebbiunga, Middle Dutch ebbe, Dutch eb, German Ebbe), from PIE root *apo- "off, away." Figurative sense of "decline, decay, gradual diminution" is from late 14c. Ebb-tide is from 1776.
Old English ebbian "flow back, subside," from the root of ebb (n.). Figurative use in late Old English. Related: Ebbed; ebbing.