grammar - Verb or Adjective?

It is both an adjective and a verb at the same time, as participles normally are.

Externally, it is an adjective, in that it can be governed by a head noun, just like an adjective: I was angry, I was disappointed (the head noun modified by it is I).

Internally, it is a verb, in that it comes from a verb and it can have some of the same arguments as verbs: I was disappointed by her behaviour etc.