Green’s Dictionary of Slang

pie adj.1

1. [late 19C+] of actions or inanimate objects, very easy, easily achievable or attainable [abbr. of easy as pie under easy as... adj.].

2. of individuals, susceptible, easily conquered, a ‘pushover’.

In phrases

old pie (adj.)

1. [mid–late 19C] (US) experienced.

2. [late 19C–1900s] (Aus./US) small-time, insignificant, second-rate.

3. [late 19C] (US) very kind.