The common definition in motorcycling for 'SQUID' is a younger rider, with no fear of death or injury, no concern for others on the road, and riding in a manner which will result in death or injury pretty soon! I think there's also an element of a 'lack of common sense', also.
Supposedly it's a combination of 'squirrely kid'.
(My interpretation....)
Another definition which you rarely see, but I prefer, and kind of arrives at the same result, is:
A rider who is riding in a manner, with, or without safety gear, that will inevitably result in him/her attaining the BONELESS, gelatinous body type of a SQUID.
Of course, SQUIDLY is how a SQUID acts!
Sometimes sailors in a Navy town are referred to as 'SQUIDS' by locals, but it isn't bike related.