To jack (steal) someone elses expression(s). The person who always Jack-spressions is usually too lame to come up with their own expression.
Emily told Leah and Moe that she was crushing on Conan O'brien. Leah squinched her nose and formed and O with her lips. Moe Jack-spression(ed) and repeated the same reaction.
by One of the SMB's June 15, 2011