"(away) from here," late 13c., hennes, with adverbial genitive -s + Old English heonan "away, hence," from West Germanic *hin- (source also of Old Saxon hinan, Old High German hinnan, German hinnen), from PIE *ki-, variant of root *ko- "this," the stem of the demonstrative pronoun (see here).
The modern spelling (mid-15c.) is phonetic, to retain the breathy -s- (compare twice, once, since). Original "away from this place;" of time, "from this moment onward," late 14c.; meaning "from this (fact or circumstance)" first recorded 1580s. Wyclif (1382) uses hennys & þennys for "from here and there, on both sides."