The name Semi-Slav suggests that somehow it's half of a Slav. How did this name come about? Or what does it mean? The two openings seem completely different to me due to the fate of the c8 bishop.
It simply because
With Black advancing pawns to both e6 and c6, the opening resembles a mixture of the Orthodox Queen's Gambit Declined (QGD) and the Slav Defense.
If you consider that there are two components to the mainline Slav - protect the d5 pawn with c6 and then develop the light-squared bishop to f5/g4 - then the semi-Slav only shares one of these two features.