What is the meaning of Ill-formed?

5-letter words containing

• 5-letter words containing d

• 5-letter words containing k

• 5-letter words containing m

• 5-letter words containing u

• 5-letter words containing t

5-letter words starting with

• 5-letter words starting with o

• 5-letter words starting with i

• 5-letter words starting with u

• 5-letter words starting with f

• 5-letter words starting with r

5-letter words in the middle

• 5-letter words in the middle e

• 5-letter words in the middle p

• 5-letter words in the middle a

• 5-letter words in the middle w

• 5-letter words in the middle s

5-letter words ending with

• 5-letter words ending with h

• 5-letter words ending with a

• 5-letter words ending with c

• 5-letter words ending with m

• 5-letter words ending with w

5-letter words excluding

• 5-letter words excluding r

• 5-letter words excluding a

• 5-letter words excluding u

• 5-letter words excluding o

• 5-letter words excluding i