masc. proper name, from Old French Guillebert (from Old High German Williberht, literally "a bright will") or Old French Gilebert, from Gisilbert, literally "a bright pledge," from Old High German gisil "pledge," a Celtic loan-word (compare Old Irish giall "pledge") + beorht "bright" (from PIE root *bhereg- "to shine; bright, white."). It was the common name for a male cat (especially in short form Gib) from c. 1400 (see Tom). As a unit of magneto-motive force, it honors English physicist William Gilbert (1544-1603). For the Gilbert Islands, see Kiribati.