1. not as alert/focused/productive as one normally is
just not with it today
If somebody is 'with it' they are considered fashionable, trendy and know about new things. Someone who is 'not with it' is the opposite, not trendy or fashionable. If you say someone is 'not with it' you may also mean that they are not alert or aware and therefore in a position where they fail to understand things properly. If you are feeling sick and drowsy for example you might tell your colleagues "I'm not very with it today, sorry." This is a very informal adjective and the opposite of 'being on the ball'.