1
general::
Put money in a safe place for future use, as in I've got about $2,000 socked away for a new car. This usage presumably alludes to putting one's savings in a sock. [Colloquial; first half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): sock something away
to place something, such as money, into reserve; to store something in a secure place. • I try to sock a little money away each month for my vacation. • I will sock away some money.
McGrawhill's American Idioms And Phrasal Verbs