someone who is only knowledgeable within the field in which they specialize; a fachidiot

専門バカ: specialist fool

馬鹿: stupid, foolish

馬鹿: idiot, fool, silly person, dolt; jerk, insensitive person; stupidity

馬鹿: huge, very, absurd

馬鹿: enthusiast, freak

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

to know, be aware of / to learn, acquire, / to be acquainted with, to know (someone) / (colloquial, in the negative) to care in the least about, to be concerned with,