It's not -tig but just -ig -- for example, you have lustig from Lust but e.g. eilig "hurried" from Eile "hurry".
As ShariniSen wrote, it's similar to the -y ending in English which can turn nouns into adjectives.
So similarly -ig doesn't have a meaning of its own; it's just an ending for some adjectives.
What about pronunciation? So far I've noticed that all words ending in "-ig" are pronounced as a German "ch" and not like an English "g" (like pig, big or wig for example). In this case "wiCH-tIG" are pronounced equal in both syllables. Is this correct? Should all -ig endings be pronounced as German -ch or can I still correctly pronounce it as an English -ig?
Wichtig, Fertig Dreißig are other examples.
Yes, the -ig ending is pronounced as -ich in standard German if it as at the end.
But if anything comes after it, that is no longer true. For example, König "king" is pronounced like Könich, but Könige "kings" and königlich "kingly" are pronounced as Könige, köniklich.