means A cold, sweet drink made by blending milk, ice cream, and flavorings until thick and frothy.

from A plain compound of 'milk' and 'shake' — the second part doing the heavy lifting, since the earliest milkshakes (late 19th century) really were shaken by hand. The word originally described boozy, eggy concoctions before ice cream muscled in and made it the soda-fountain staple we know. 'Shake' itself is ancient Germanic, from Old English 'sceacan,' to move briskly to and fro — which is exactly what a barkeep did to make one.