Yes, plumb is a Scrabble word!

Adverb

1. Completely; used as intensifiers

2. Conforming to the direction of a plumb line

3. Exactly

Verb

1. Measure the depth of something

2. Weight with lead

3. Examine thoroughly and in great depth

4. Adjust with a plumb line so as to make vertical

Adjective

1. Exactly vertical

Noun

1. The metal bob of a plumb line