This is an expression I picked up in a Hugh Kelsey book. It means that your mind wandered off to something you saw outside the closest window (like a flying cow ;) and you just dropped the ball. That's what happened to Frederic in this 6NT on deal 3 from the final match against Aronov-Mihov:
We reached 6NT after a strong club auction where South showed hearts and dummy diamonds and clubs. A low club lead to the J and Q followed by a diamond floated around to the 9. Diamond continued to Q and ace and when entering the hand after unblocking the hearts, RHO discarded a spade. The deal is now an open book and this is the remaining cards:
This is a classic double-squeeze position and and cashing the hearts means West must release the spade guard and dummy can discard the 8 of clubs on the final H honor. After a club to the K, East is finished and dummys small diamond or the 9 of spades is the slam-going trick.
Then came the 'cow', and declarer called for the last diamond instead of the club on the final H. Endposition ruined and -50 on the scorecard. What we didn't know was that declarer went 2 down at the other table in the same contract.
Frederic is a very good cardplayer; things like this happen to even the best. I'll show you a deal later where I fouled out, but first two more crucial deals from the very last match.
Beware of the cows.