lol! can be written as 101!
Mathmatically 101(binary) is 5(decimal). Therefore 101!(binary) is 5!(decimal)
The notation '!' is called factorial. 5! (factorial of 5) means 1*2*3*4*5 which is equal to 120. Since 120 is written in decimal form, a 'd' is added to the end of it to form 120d.
That's why 120d means lol!
(note: 120d has the exclamation mark included, 120d would mean lol!! with two exclaimation marks)