An accessible store of money, intended for small purchases.
The term 'petty cash' derives directly from the word 'petty' meaning 'minor'; 'small'; 'of secondary importance'. 'Petty cash' was a small amount of cash that was kept aside for trifling purchases, too small to require the effort of the making out and cashing of a cheque.
The expression is first found in print in the English writer Roger North's The Gentleman Accomptant [Accountant], 1715:
A school-Boy, bid to accompt for his petty Cash, will naturally fall in to do it so.
Petty cash was commonplace enough by the 19th century for 'petty cash books' to be so named. The first reference to such is in The Times, June 1827:
Mr. Blount here read some items from the petty cash book.