Adjective; Variation in spelling and linguistic pronunciation of the word "serious", but has the exact same meaning.
It can also be used as a modifier of another adjective, most interestingly: serious. See example below.
In some modern applications, the variation "Srsly" has the same effect, only it is done by disemvoweling (removing of the vowels in a word).
If you're aware that the letter "y" is sometimes a vowel, kudos
Sirius was surriously serious about serial killing that cereal.