walk·ing-a·round mon·ey   [waw-king-uh-round]
noun
1. money that is carried on the person for routine expenses and minor emergencies; pocket money.
_{Source: Dictionary.com – definition of “walking around money”}

walk·ing-a·round mon·ey  
noun
1. money that is carried on the person for routine expenses and minor emergencies; pocket money.
_{Source: Dictionary.com – definition of “walking around money”}

The idea is that it's enough money to take care of minor needs during a routine footwalk. (If you get thirsty, you at least have enough money buy a drink, for example.)

Another phrase with a similar meaning is pin money. From NOAD:

pin money
noun

1. a small sum of money for spending on inessentials.

walk·ing-a·round mon·ey [waw-king-uh-round] Show IPA noun 1. money that is carried on the person for routine expenses and minor emergencies; pocket money. 2. Also called street money. Political Slang. cash sums given by political managers, district leaders, or the like, to grass-roots workers and others for expenses incurred while canvassing for votes or doing other chores before an election. Origin: 1975–80, for def 2 Source Dictionary.com

walk·ing-a·round mon·ey   [waw-king-uh-round]
noun
1. money that is carried on the person for routine expenses and minor emergencies; pocket money.
_{Source: Dictionary.com – definition of “walking around money”}