1) A pool or marshy spot.
The word occurs in very early minor place-names: 1220-8 una dimidia acra terre in Flasc, Great Ouseburn
n.d. Terra de feodo Abbatis [of Selby] in Northduffeld. In primis in Waterflaskes, computando unum selionem
1407 ‘three selions lay in Watirflasche’, North Duffield
1472-3 de uno clauso vocato Flask, North Stainley. Several such place-names survived and the spelling evidently interchanged with ‘flash’. Flask House in Hartwith is such an example: 1677 John Benson senior de Flashehowse, Hampsthwaite.