from Wiktionary, Creative Commons Attribution/Share-Alike License
- suffix Forms the past tense and/or past participle of some verbs (leapt, kept, dreamt, etc)
- suffix An excrescent ending appended to words suffixed with -s.
From Middle English -te (preterite ending), -t (past participle ending), from Old English -de (first and third person preterite ending), -d (past participle ending), from Proto-Germanic *-id- (preterite stem ending of class 1 weak verbs) and Proto-Germanic *-idaz (“past participle ending of class 1 weak verbs”). (Wiktionary)
Probably resulting from -s + the, or various other words beginning with th-, t-. (Wiktionary)