誰かのひじが私の背中に当たった。

who

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

elbow

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

I; me; private affairs; personal matter; secrecy; selfishness

back (of body)

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

to be hit; to strike; to touch; to be in contact; to be affixed; to be equivalent to; to be applicable; to apply to; to be right on the money (of a prediction, criticism, etc.); to be selected (in a lottery, etc.); to win; to be successful; to go well; to be a hit; to face; to confront; to lie (in the direction of); to undertake; to be assigned; to be stricken (by food poisoning, heat, etc.); to be afflicted; to be called upon (by the teacher); to treat (esp. harshly); to lash out at; to be unnecessary; to be hitting well; to be on a hitting streak; (in fishing) to feel a bite; (of fruit, etc.) to be bruised; to spoil; to feel (something) out; to probe into; to check (i.e. by comparison); to shave; to be a relative of a person; to stand in a relationship