HY: hydra
The only relevant dictionary entry I could find was for the abbreviation “thx”.
Similarly:
HZ = Hochzoll
IA = liability
IB - feasible
IC = ichthyosaurus
ID - wide
IE: satisfied
IF - if
IG: big
IH - nihilist
II: radii
ij - hijack
IK = bikini
il - illness
IM: impudently
IN - inning
IO = motion
[del]IO: Io[/del]
(I liked Io!)
IP: lip
IQ - antique
IR - hirsute