The meaning I have found so far is "foder" = meaning to feed the animals + "gill" = meaning a meadow or valley area
for a slightly different meaning go to:http://www.angelfire.com/co2/fothergill/history.html
"Researchers are fairly certain that the name 'Fother' is an old Scandinavian personal name. Bardsley in his work Dictionary of English and Welsh Surnames (1901) adds that 'gill' refers to where he settled. In the Old Norse language 'gil' refers to a wooded glen, small ravine or brook. The Norwegians settled in and around Cuerdeale (Kendal, Westmorland County, England) between 865 and 955 AD. Though it is not known just when the name Fothergill came into being, it can be assumed that it was used to distinguish a man from the area in which Fother (whomever he may have been) had settled. Bardsley adds 'That Mallerstang Forest and Ravenstonedale in county Westmorland are the homes of this name, there can be no doubt'. "