"Amusing".

This is what's great about Arabic; as you progress in the language, you'll start to get more familiar with the templates that the roots fall into. This particular one is the adjective for verbs in Form IV – causative verbs which use the template aCCaCa. In this case, the verb is "أَضحَكَ" (aDH7aka – to make laugh). The active participle, which is used for the adjective too, uses the template muCCiC – so, here, it's "مُضحِك" (muDH7ik – s.o./sth. that makes (s.o.) laugh), and then of course with ta' marbuta, as it's the feminine form (مُضحِكة - muDH7ika).

For further reference, a couple other verbs and their participles are:

"أَحزَنَ" (a7zana – to sadden)
"مُحزِن" (mu7zin – sad; saddening)

"أَزعَجَ" (az3aja – to annoy)
"مُزعِج" (muz3ij – annoying)

"أَفرَحَ" (afra7a – to delight s.o.)
"مُفرِح" (mufri7 – delightful)