Something done expressly is done only for a very specific reason or purpose. Growing up, your grandparents' formal living room might have been used expressly for adult parties.
A politician running for office might use campaign donations expressly for buying television ads, and a middle school student might use her school laptop expressly for chatting with her friends and posting photos online. In both cases, there is one very specific intention. The earliest, fourteenth century meaning of expressly was "in detail" or "plainly." By 1600, it came to mean "for the express purpose," from the Latin expressus, "clearly presented."