The naming “Sportster” comes from the English vocabulary. Member of the sport organization – that would be the most suitable explanation of that term. In fact in Britain The Sportster would be a name of a young 18 – or yearly 19 years of age “wealthy man of leisure who frequented sporting events such as horse-racing” (wiki)  

There are quite a few airplanes designed and produces with the name of “Sportster”. The most famous of cause the Great Gee Bee Sportster plane from Granville Brothers Aircraft of Springfield MA Total of 14 great historical airplanes were built between 1929 and 1934 as aviation golden age air racers.

In 1932 her first flight Kinner Sportster Model K-1 was in the air. In according to wiki Kinner Sportster K-1 and B-1, 1933, with a 5-cylinder radial engine of 100 horsepower (75 kW) to 125 horsepower (93 kW). Became rather popular and sold in some dozen pcs. A few of them still flying. Sportster K is an earlier version of the B1 with the only difference being it was powered by the 100 hp (75 kW) Kinner K-5 engine. The Sportster B-2 had the wings clipped to 35 ft and was powered by the 125 hp B5 engine. After Bert Kinner was forced out - he founded the Security Aircraft company and produced the Security Airster which was a very close copy of the Sportster. Timm purchased the rights to and manufactured several Sportsters under the Timm name. Later the airframe design was used by the Call-air corp. and finally design elements of the Sportster can be found in the Piper Pawnee ag aircraft.

About the same time in late 1930 s Rearwin Aircraft & Engines started production of 330+ upper wing close cabin two sitter monoplanes that we know as Rearwin Sportster

Martin Hollman has designed the first home built gyroplane that was able to carry a passenger called Sportster in 1974. 65 of those machines were produced by ADI thru 2005

And now this great name comes to the 1990-s. In 1996 the first wood wing Dana Axelrod  Sportster was flying hand made from his garage In That plane was the first in series of 10 great Sportsters produced by Warner Aerocraft of Beverly MA

In 1998 Theiss Aviation of Salem Ohio produced two home built biplanes that we know as Theiss Sportster.