From an unused root meaning to depress; a basin (as deepened out) -- bowl, dish.
Englishman's Concordance
1) bowl, basin Part of Speech: noun masculine A Related Word by BDB/Strong’s Number: from an unused root meaning to depress Same Word by TWOT Number: 1536
Brown-Driver-Briggs
סֵ֫פֶלnoun [masculine]bowl (√ unknown; perhaps foreign word; Assyrian saplu, bowl, basin (DIHWB 508 SchrCOT 2 Kings 9:2) [occurs as object of tribute or plunder]; Late Hebrew=Biblical Hebrew; ᵑ7סִיפְלָאbowl, basin, Christian-Palestinian Aramaic (wash-)basin SchwIdioticon 64; Arabic bowl, jar FlLevy NHWB iii,320; Kl. Schr.ii. 556 f. is perhaps loan-word Frä67f.); — absolute ׳מְלֹא הַסּJudges 6:38; for drinking, construct סֵפֶל אַדִּירִיםJudges 5:25a bowl of (fit for) nobles, huge bowl (GFM).
Judges 5:25 HEB:חָלָ֣ב נָתָ֑נָה בְּסֵ֥פֶל אַדִּירִ֖ים הִקְרִ֥יבָה NAS: In a magnificent bowl she brought KJV: butter in a lordly dish. INT: milk gave bowl A magnificent brought
Judges 6:38 HEB:הַגִּזָּ֔ה מְל֥וֹא הַסֵּ֖פֶל מָֽיִם׃ NAS: from the fleece, a bowl full KJV: out of the fleece, a bowl full INT: the fleece full A bowl of water