1) Euphemism for a penis, where "one-eyed" refers to the external urethral orifice (or
pee-hole) located at the tip of the penis' head.
2) One-eyed willy, both alternate spelling and definition.
Expression's origins: derived from the name
popular character from the film "Goonies" (although this context has been permutated with
time). See also dong, dick, schlong, willy, wang, johnson, cock.