Tu visada prisigalvoji daug pasiteisinimų. You always make up a lot of excuses. Jis nieko nesuprato, todėl prisigalvojo galimų paaiškinimų. He didn't understand anything so he made/thought up (imagined) some possible explanations. Ji dažnai prisigalvoja visokių nesąmonių. She often "invents" various nonsenses. The word "prisigalvoti", as you can see from the examples, is almost always used in a bit negative or ironic sense, i.e. when a person doesn't know something for sure, he/she can "prisigalvoti" lots of differents explanations, which usually aren't true. "Prisigalvoti" is something like using your imagination to think things up. Another example. Jis labai gerai suprato šį procesą ir todėl sugalvojo jam paaiškinimą. He understood this process very well and he made up an explanation for it. Jis visiškai nesuprato, kaip vyksta šis procesas, todėl prisigalvojo daug įvairių paaiškinimų. He didn't have a clue about this process, so he made up a lot of explanations for it. I think the last example will give you the best idea of what the word "prisigalvoti" means. An interesting note - usually the word "prisigalvoti" means thinking more than one thing (explanations, nonsenses, excuses) up. I hope that it helps.