From an unused root meaning to break apart; fracture, i.e. Severity -- cruelty, rigour.
H6531. perek
מֶּ֫רֶךְ noun [masculine] harshness, severity; — in phrase ׳בְּפ Leviticus 25:33, בְּפָ֑רֶךְ 25:43; 25:46; (H P), Ezekiel 34:4 (all with רדה rule harshly); Exodus 1:13 (with וַיַּעֲבִדוּ), 1:14 (with עֲכֹדָה; both P).
II. פרך (√ of following; compare Assyrian parâku, bar, shut off; parakku, apartment, shrine; Syriac shrine (see RSJPhil 13, 283); see also Muss-ArnJBL xi (1892), 77 Köii. 1, 201).