The sign for "bold" varies depending on what you mean.
A very common way to sign "bold" is to touch your forehead with your index
finger and then form both hands into "S" hands and "whack" your non-dominant
hand with the dominant hand with a downward glancing blow.
If you mean bold as in "brave"
then see: BRAVE
The other day I was at a meeting (oh the joy) and my interpreter signed
"thick" while mouthing the word "bold." This particular interpreter is one
of the best in the Sacramento region and her signing is consistently
excellent. I replayed her last few signs in my mind and realized that the
speaker had been talking about some sort of document and that she was
referring to "bold" as in "bold typeface." Looking back I don't
recall a thing about what was discussed in the meeting (heh) but I certainly
remember such a clever use of the sign "THICK."
THICK-(general_version) + "context" = "boldface type"



Note: Make sure to use a "claw" hand so as to distinguish this sign from
CELL phone. Doing THICK a bit lower on the
cheek also helps distinguish it. And notice my mouth? I'm actually
forming my teeth and tongue as if I were to make
the "TH" sound (such mouth phonemes and morphemes are very common --
even required in many ASL signs. They are not Englishisms but rather the
result of living in a diglosic society.).
Notes:
There are quite a few other ways to sign "bold" and eventually I'll get
around to posting them. (If you need to know a certain "version" feel free
to donate a "bold" amount of money so as to free up my time.)
There is a
version that points to the temple (forehead) and adds the sign
PROCEED. The sign PROCEED (in this use)
is modified by doing a very strong, fast movement.
Another way to sign "bold" is to use the sign for "GUTS" which is done by
using a bent hand (palm up and/or towards your stomach) and poking your guts
twice (just under the ribcage). When I do that version of "bold" I tend to
do an "ooh" mouth shape with an extended jaw and a slight shake of the head
as if thinking "oh wow that takes a lot of guts."