A unit of measurement used for determining the size of one's penis. Similar to the 'guage' system, Galloways operate by calculating how many of one's penises fit the area of one S. Galloway penis which is approximately 10 inches long and 2.5 inches wide.
This is represented by whole numbers when said penis is less than and equal to one Galloway penis. When said penis is larger than one Galloway, fractions represent the area of said penis that is taken up by one Galloway penis.
Boy: Mine is three Galloways!
Girl: ...Are you joking?
Boy: Mine is one quarter Galloways!
Girl: Holy moly that's huge!!