Raisons D'étre, meaning Reason for Being in French.
"[...]which is a song by The Beatles, who I love, because entomology is one of my raisons d'étre, which is a French expression I know." p.1
"I love making jewelry for her, because it makes her happy, and making her happy is another one of my raisons d'étre," p.7
"[...] how Buckmister just sleeps and eats and goes to the bathroom and has no raison d'etre[...],", p. 42
"That night when I decided that finding the lock was my ultimate raison d'etre-the raison was the master over all other rasions" (p.69)
"I knew I could never let Mom hear the messages, because protecting her is one of my most important raisons d'étre, so what I did was I took[...]" (p.68)
"I didn't want to hurt her feelings, because not hurting her feelings is another of my raisons d'etre." (100) (Oskar is talking about this Grandmother)
French is the language it's in...
Oskar's necessity for raisons d'être displays a lack of direction in life, perhaps resulting in his erratic thoughts as discussed in "amorphous thought process of young children."
Oskar's need to assign meaning to his existence connects to the idea that life is meaningless