"Furtive"

When:

Lookups on Merriam-Webster started spiking around May 2, 2010.

Why:

After the failed bombing in Times Square, New York City police commissioner Raymond Kelly described the surveillance video of the suspect in a widely quoted statement:

"He looks around, takes off that shirt, puts it in a bag and continues to walk south, looking a couple of times in a furtive manner."

The word furtive means "done by stealth : surreptitious"; its roots go back to the Latin word for thief.