Thingamajig, doohickey, whatchamacallit, thingamabob &c

1. What do you call items whose name or use is forgotten?
2. What kind of word is whatchamacallit?

What do you call items whose name or use is forgotten?

"Gizmo" is used for mechanical, electrical or technological devices which either lack a name or the name can't be recalled.

What kind of word is whatchamacallit?

a metasyntactic term used for any object whose actual name you don't know or can't remember.