The only free and comprehensive online etymological dictionary of the Spanish language
in- (1), im- A negation prefix indicating "not" or "against."
From Latin in- 'id.' From Proto-Italic *in- 'id.' From Proto-Indo-European *n̥- 'id.'
Indo-European
Italic
Oscan an-, Umbrian a-
Celtic
Old Irish in-, Old Welsh an-, Old Breton an-, Old Cornish an-
Germanic
Gothic un-, Old Norse ó-, Old High German un-, Old Saxon un-, Old English un- (English un-)
Hellenic
Ancient Greek ἀν- (án-)
Indo-Iranian
Sanskrit an-, Avestan a-
Tocharian
A an-, B an-