If you mean "organ" as in a type of
musical instrument" then "mime" the
playing of an organ. Or spell it and then do the mime. If the
difference between a piano and an organ is immaterial to your story
or conversation then just sign "piano." If the difference is
important then you could sign PIANO but do one hand up higher and
the other lower as if showing a multi-keyboard type organ.
PIANO:
Notes:
See: INSTRUMENT
Notes: