Quilp goes to his lawyer's office but lawyer is out, and both clerks are out. So he leaves a message, with the very small seldom seen female servant watching him lest he steal even "a wafer". Definitin 5 seems to fit.
wafer ['weýfə]
noun
1 a thin crisp sweetened biscuit with different flavourings, served with ice cream, etc.
2 (Christianity) a thin disc of unleavened bread used in the Eucharist as celebrated by the Western Church
3 (Pharmacol) an envelope of rice paper enclosing a medicament
4 (Electronics) a large single crystal of semiconductor material, such as silicon, on which numerous integrated circuits are manufactured and then separated
5 a small thin disc of adhesive material used to seal letters, documents, etc.
verb
6 [transitive] to seal, fasten, or attach with a wafer
[ETYMOLOGY: 14th Century: from Old Northern French waufre, from Middle Low German wafel; related to waffle1]
'wafer-"like, 'wafery adjective
