According to Urban Dictionary, being “caught lacking” means "being caught by the enemy without a weapon to defend yourself."

What does it mean if someone is lacking?

1 : to be deficient or missing time is lacking for a full explanation. 2 : to be short or have need of something he will not lack for advisers The area does not lack for good restaurants. transitive verb. : to stand in need of : suffer from the absence or deficiency of lack the necessities of life She lacked confidence ...

What does caught slacking mean?

to work more slowly and with less effort than usual, or to go more slowly: Everyone slacks off/up at the end of the week. disapproving You'll be in trouble if you're caught slacking on the job. Slack off your speed as you approach the corner.