Like all prepositional compounds ending in לְ־(l'-), שֶׁלְּ־ undergoes vowel modifications, and merges with a following definite article; see usage notes atלְ־.
Etymology
From שֶׁ־(she-, “that, which”) + לְ־(l'-, “to”). Attested since Mishnaic Hebrew. More recently supplanted by the standalone form שֶׁל(shél), but the inflected forms are unchanged.