Seriously I was baffled when one of my best students asked me what "тупить" meant in English?
My head started throbbing, oh god, I wasn't prepared for that!!!! I actually started тупить'ing myself.
I couldn't come up with anything better than "I'm acting a dumbbell." and "I'm being slow-witted". What the hell? I smoked nervously during the break, and the growing rubbing pain in my head seemed to drill my whole body like some mad drunk construction worker ... блин ... как же отыскать перевод .... I somehow managed to conceal the pain and disappontment till the end of the class ,,,, Then I bought a Золотая бочка в баночке и залил Горе (фрикативный Г). In a word, How do I find the right equivalent to "тупить, я туплю"????

off the top of my head

off the top of my head .......

being slow on the uptake OR slow on the draw (to be slow to understand new ideas)

Example 1: I tried to explain the new database, but they were remarkably slow on the uptake. (CIDI)

Example 2: You're a bit slow on the draw aren't you? Can't you see the joke? (CIDI)

[slow on the draw]
(be) a bit obtuse (a little dull and stupid; incapable of understanding; a bit thick; slow-witted; a little too dumb to understand)

Example 1:...I didn’t understand you. I must be a bit obtuse. Can you explain it to me again?
Example 2:Being obtuse is no excuse. If you are too dull-witted to do this job, we’ll get someone to replace you.
Example 3:You must be a bit obtuse if you can’t even understand how to follow simple orders and directions.

(be) a little/ a bit on the slow side

draw a blank (to mean that you forgot sth you should have known, perhaps a name or a phone number)

Example 1: Help me out. I’m drawing a blank. —
Example 2:

(have) a senior moment (a lapse of memory in an older person) — типа, склероз замучал; эклер в голове
I had a senior moment and forgot your name. Sorry.

(have) a blond moment (a lapse in thinking, something like a senior moment; a flash of momentary stupidity)
Sorry. I was having a blond moment. And that’s hard for a redhead.

see also

If I think of any more, I’ll post here.
Thanks for dropping in,

Thanx Fenya, it's getting

Thanx Fenya, it's getting clearer. ;)

And how about 'But why?'

And how about

'But why?' said Charles, being deliberately obtuse.
Perhaps I’m being obtuse, but what has all this got to do with me?

I think being obtuse implies

I think being obtuse implies a deliberate action.