The ending -iadur, -adur indicates something, someone, or often a book, that does something.

Here it refers to a cyfrifiadur (a 'counting thing' = a computer) working on someone's glin (lap) or pen-glin (knee). Mash them together and shorten things a bit to coin a new word gliniadur - a 'lap computer' = a laptop.

Similarly:

• dyddiadur
• gwyddoniadur
• geiriadur
• ffoadur