The root word here is the verb 'kushinda' which means 'to win'. When you conjugate the verb, 'ku' is dropped (most verbs will do this), then 'u' is added as the subject for 'you', 'me' in the present perfect tense for 'have' and 'je' is an ending for the verb to ask 'how?'
So broken down, the phrase is literally asking 'you, how have you won?', but in practice is used to ask how a person's day has been (have they won today?)
Should be "is" ... the day is still going on .... -me- is in swahili more than that here is suggusted... it is also for ongoing activities. If someone is know to arrive soon they will say .... amekuja ..... if he is there.... amekuja..... later if someone wants to know if he has arrived it is .... amekuja...