There is no English equivalent. A direct translation is "(subject) can't read the air (or atmosphere). However it is used in two contexts. 1) when someone says or does something out of context of the situation around himself/herself, he/she is k.y. (i.e. a random, spontaneous comment on an unrelated subject). 2) it can be used when someone is lagging in the conversation, when that person brings back a subject that already passed.