Skype is usually used for voice and video calls, but you can deliver text-based messages via Skype as well. A pending message in Skype is one that hasn't been delivered to its intended recipient yet. It will only be delivered when both you and the other user are online simultaneously. Otherwise the message will be stored on your computer until it's able to be delivered. You can tell a message hasn't been delivered because a rotating circle displays to the right of the text.
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The pending message is stored locally on the computer you're currently logging into Skype from. Unless you're signed into Skype from that specific computer when the recipient logs in, it will not be delivered.
Pending While User Online
Skype cannot always deliver messages, even if the intended user is online. This can be due to a connection problem on your end, or on the recipient's. Network congestion is a common cause for the delay of the message; you'll see the status become pending. It will be delivered as soon as the congestion clears.
You need to be aware that when a message becomes pending, it will be delivered the moment the intended recipient logs on. To the recipient it will appear as though you have just sent the message. Even the time stamp of the message will be the delivery time.
Modifying Pending Messages
Since a pending message has yet to be sent, you can edit or remove it entirely. Simply right-click your message and select "Edit Message." The text appears in the text box below and you may edit it as you see fit. If you wish to prevent your message from being sent, you can select "Remove Message" instead.