a.k.a.

also known as

approx.

approximately

ASAP

as soon as possible

Attn.

for the attention of

b/c

because

BTW

between or by the way

c/o

in care of

exc.

except

FYI

for your information

FYA

for your action

PA

personal assistant

p.a.

per annum (per year)

Pto.

please turn over

viz.

videlicet, namely

他にも沢山の頭字語などがあります。 Acronym Finderなども利用しましょう。