The hashtag "WCW" on Instagram and other social media sites or applications stands for "Woman Crush Wednesday." This hashtag is used when uploading a picture of an attractive woman.
WCW is commonly applied to friends and celebrity idols. It is a tag that's mostly used on Wednesdays, but some apply it to photos on other days of the week. The photos used are a mixture of selfies and professionally taken photos of famous people. Sometimes the WCW tag is used as a joke with a picture of someone who is traditionally unattractive or with an unflattering picture of a friend.