Mongrel
late Middle English: of Germanic origin, apparently from a base meaning ‘mix’, and related to mingle and among.
wiktionary
From Middle English mongrel, equivalent to mong(“mixture”) + -rel(pejorative diminutive); from Old English ġemong(“mingling”) (whence Modern English among), from Proto-Germanic *mang-(“mix”). [1]
etymonline
mongrel (n.)
mid-15c., "individual or breed of dog resulting from repeated crossings or mixture of several different varieties," from obsolete mong "mixture," from Old English gemong "mingling" (base of among), from Proto-Germanic *mangjan "to knead together" (source of mingle), from a nasalized form of PIE root *mag- "to knead, fashion, fit." With pejorative suffix -rel.
The distinction between a mongrel and a hybrid (a cross between two different breeds) is not always observed. Meaning "person not of pure race" is attested from 1540s. As an adjective, "of a mixed or impure breed," from 1570s.
