Di means without/less. Here are examples of it in use.

calon               heart                >          digalon                        downhearted/depressed

gwaith              work                >          diwaith                        unemployed

lliw                   colour              >          di-liw                           colourless

clem                clue                 >          di-glem                        clueless (colloquial)

priod                married           >          di-briod                       unmarried

cartref             home               >          digartref                      homeless

galw am          call for             >          di-alw-amdano            uncalled for (it)

asgwrn cefn    back bone       >          di-asgwrn-cefn           spineless

pen draw         end                 >          di-ben-draw                endless