The story
From self- + soar. Many intrepid flyers joined the 70s' Self-Soar Association [S-SA] that had "Otto" memberships and membership numbers in honor of Otto Lilienthal who self-soared off a hill he had built. The booklet series Low & Slow and the Hang Glider Weekly (together 216 editions) frequently used the term self-soar to describe flying using an unpowered glider flown after launching by some system without the aid of motors or other persons.
Etymology from Wiktionary, CC BY-SA 4.0.