Chris Templeman acknowledged that there were no excuses as 10-man Forfar lost ground in the race for a League One promotion play-off spot.
The Loons are now six points adrift of a top-four position after their New Broomfield humbling as their hosts moved closer to safety with a resounding performance.
Three goals in the space of eight first-half minutes left Forfar with a mountain to climb and it was practically mission impossible when Iain Campbell was sent-off.
On-loan Dundee United forward Dale Hilson’s 10th goal of the season reduced the deficit but the Diamonds struck twice in the second half to put the game out of reach.
“We’re obviously disappointed with the result and the performance,” said Templeman after Forfar’s four-game unbeaten run was brought to an end.
“You can’t really put it down as one of those days as we were really poor. We didn’t play well.
“We didn’t start the game well and were punished for losing three early goals which were all preventable.”
Airdrie took the lead when Gregor Buchanan headed home Nicky Cadden’s corner and Jamie Bain made it 2-0 with a howitzer into the top corner of the net.
Mark Baxter diverted Jim Lister’s ball off the post and into his own net before Campbell received his marching orders for a high challenge on Bain.
“I thought it was a harsh sending-off and that made life very difficult for us,” added Templeman.
Forfar pulled a goal back before the interval thanks to Hilson’s run and finish but their joy was to be short-lived.
Lister grabbed Airdrie’s fourth when his header, from a Paddy Boyle cross, deflected into the bottom corner of the net. And the points were wrapped up in style when Craig Barr’s pass found midfielder Liam Watt who took the ball round keeper Darren Hill and slotted home.
“Christmas has come early for Airdrie and they were worthy winners in the end,” said disappointed Forfar manager Dick Campbell.
“I didn’t think it was a red card but everything that could have gone wrong for us did go wrong.”