float (v.) Look up float at Dictionary.com
late Old English flotian "to float" (class II strong verb; past tense fleat, past participle floten), from Proto-Germanic *flotan "to float" (cognates: Old Norse flota, Middle Dutch vloten), from PIE root *pleu- "to flow" (see pluvial). Of motion through air, from 1630s. Related: Floated; floating.
float (n.) Look up float at Dictionary.com
early 12c., "state of floating" (Old English flot meant "body of water"), from float (v.). Meaning "platform on wheels used for displays in parades, etc." is from 1888, probably from earlier sense of "flat-bottomed boat" (1550s). As a type of fountain drink, by 1915.
Float.--An ade upon the top of which is floated a layer of grape juice, ginger ale, or in some cases a disher of fruit sherbet or ice cream. In the latter case it would be known as a "sherbet float" or an "ice-cream float." ["The Dispenser's Formulary: Or, Soda Water Guide," New York, 1915]



Few soda water dispensers know what is meant by a "Float Ice Cream Soda." This is not strange since the term is a coined one. By a "float ice cream soda" is meant a soda with the ice cream floating on top, thus making a most inviting appearance and impressing the customer that you are liberal with your ice cream, when you are not really giving any more than the fellow that mixes his ice cream "out of sight." ["The Spatula," Boston, July, 1908]