Meaning: to sleep is a strong verb in the Germanic languages. While I'm quite aware that strong vs weak anything has very little bearing on modern English, this is still something that puzzles me.
Sleep and slept do show some kind of ablaut, but then where does the -t come from? Why was it added to the end of this verb? I know very little about the specifics of English etymology, and only really have the viewpoint of studying other Germanic languages.
Was it just something as simple as not wanting "slep" as a word and adding the -t made it easier to say/understand? Or just some kind of leveling to make it like wept, kept, etc.?
Best Answer
As John Lawler mentioned in the comments, sleep in present-day English is an irregular weak verb. The synchronic classification of this verb is not related to the conjugation of cognates in other languages, past or present.
Irregular weak verbs show the dental suffix characteristic of weak verbs, but also show other irregular changes; in this case, a vowel change that can be characterized (from certain somewhat abstract perspectives) as "shortening", /iː/ > /ɛ/ ("long e" to "short e"). Wept and kept are other examples of this vowel change.
I don't know enough historical linguistics to explain the diachronic development of the verbs sleep (in English) and schlafen (in German), or why one is weak and the other strong. It is not unprecedented for verbs to move between the strong and weak classes in Germanic (e.g. present-day English help is weak, even though German helfen is strong).
The OED says:
I confess that I don't understand what is meant by "the strong conjugation (with reduplicated past tense)" with respect to Old English since I only see reduplication in the Gothic past-tense form, not in any of the other past-tense forms listed in the OED article.
"Slep" is attested, but it doesn't look very much like I'd expect a strong past-tense form to look in modern English, in that I don't think the development of the Old English long vowel /eː/ to Modern English /ɛ/ would be regular in this context. The OED seems to agree; it gives the following list of forms that it classifies as strong past tense:
I would expect the regular reflex of the Old English strong past tense to be pronounced /sliːp/ in Modern English (perhaps homophony with the present-tense form was part of why it died out).