Sentence:  Joseph Jastrow , the younger son of the distinguished rabbi , Marcus Jastrow , was a friendly , round-faced fellow with a little mustache , whose field was psychology , and who <WSD>was</WSD> also a punster and a jolly tease . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)