pin money

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Money for incidental expenses.

from The Century Dictionary.

• noun An allowance or occasional gift made by a husband to his wife, either voluntarily or as a part of the marriage settlement, for her separate use, to be employed in the purchase of apparel or of ornaments for her person, or for other personal expenditure.
• noun A similar allowance made to any one, as to a daughter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, dated A relatively small sum of cash kept in one's personal possession, for routine expenses or incidental purchases; an amount of money which is not particularly significant.
• noun idiomatic, dated An allowance of money given by a man to his wife or to other dependents for their personal, discretionary use.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun cash for day-to-day spending on incidental expenses