Roomy comes from room + -y, from Old English rūm ('space, extent'), from Proto-Germanic *rūmaz ('roomy, spacious'), from Proto-Indo-European *rewh₁- ('to open up, make space'). The historical chain includes Old English rūm and Proto-Germanic *rūmaz. The quoted older senses, "space, extent", "roomy, spacious", and "to open up, make space", supply the semantic anchors for roomy. The formation is best read as suffixal and borrowed: the word roomy carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "having plenty of space; spacious". English derivatives include roominess and roomily; related English forms include room. The result is a adj whose etymology preserves both the older form and the later English specialization.