Why Q signals are three-letter shorthand from international radio practice, and QSL is the acknowledgment signal: it means the message or transmission was received and understood. Sending "QSL" in a CW contact tells the other station that the information got through correctly. The familiar QSL card carries the same idea, written confirmation that a contact was received.
Watch out The choice about having already confirmed the contact plays on the QSL card association, but the Q signal itself is an acknowledgment of reception in the moment, not a statement about past confirmations. "Send slower" is QRS.
QSL = "Sure Little" doubt: got it, understood. QRS = Reduce Speed.