An Etymological Dictionary of the German Language/fassen
fassen, vb., ‘to hold, grasp, comprehend,’ (refl.) ‘to make up one's mind,’ from MidHG. vaȥȥen, OHG. faȥȥôn, ‘to handle, seize, load, pack, arm oneself, dress, go’; it seems to be a combination of two or more really different roots. Comp. OIc. fǫt, neu. plur., ‘garments’ (Goth. *fata, ‘garments,’ may be deduced from Span. hato, Port. fato, ‘stock of clothes, wardrobe’); the West Teut. fat (see Faß), has not this meaning, but MidHG. (OHG.) vaȥȥen, ‘to dress oneself, points that way. In the sense ‘to seize,’ the word may be connected with Faß, lit. ‘engulphing,’ from which the meaning ‘to load’ would be evolved. In the sense of ‘to go’ (sich vaȥȥen, MidHG.) it must probably be connected with Fuß, or more closely with AS. fœt, ‘step.’ See Fetzen, Fitze.