"Yes! and he faintly understands, poor Mr. Toots, that they are saying something of a time when he was sensible of being brighter and not addle-brained..."

Webster's 1913 Dictionary

Definition: \Ad"dle\, n. [OE. adel, AS. adela, mud.]
1. Liquid filth; mire. [Obs.]

2. Lees; dregs. [Prov. Eng.] --Wright.


\Ad"dle\, a.
Having lost the power of development, and become rotten, as
eggs; putrid. Hence: Unfruitful or confused, as brains;
muddled. --Dryden.


\Ad"dle\, v. t. & i. [imp. & p. p. {Addled}; p. pr. & vb.
n. {Addling}.]
To make addle; to grow addle; to muddle; as, he addled his
brain. ``Their eggs were addled.'' --Cowper.


\Ad"dle\, v. t. & i. [OE. adlen, adilen, to gain, acquire;
prob. fr. Icel. ["o][eth]lask to acquire property, akin to
o[eth]al property. Cf. {Allodial}.]
1. To earn by labor. [Prov. Eng.] --Forby.

2. To thrive or grow; to ripen. [Prov. Eng.]

Kill ivy, else tree will addle no more. --Tusser.

Most common usage I have seen refers to eggs that have been shaken, so that the germinating layer has been disrupted, and the egg cannot produce a chick. As a figure of speech, someone whose mental processes have become jumbled, is said to have an addled brain.