before, ahead, previous, future, precedence
先 centers on being ahead in time or order: before, previous, or taking precedence. The sense of 'future' is a natural extension, as what lies ahead is yet to come.
point; tip; end; nozzle
teacher; instructor; master
first (of all); firstly; to begin with; before anything else
preference; priority; precedence; right of way
senior (at school, work, etc.); superior; elder; older person; predecessor
the other day; a few days ago
going ahead; going first
last week; the week before
last month
advanced; developed
head (of a line, group, etc.); front; lead; forefront; vanguard
pointed end; tip; point; cusp (of a leaf, crescent moon, etc.); apex (of a curve)
starting in advance; starting first; going first; going ahead
ancestor; forefather; foremother
head start; initiative; preemption
cutting edge; leading edge; forefront; state of the art
the head; the foremost; beginning; the very front
ancestor
for now; for the time being; for the present
fingertip; finger
arriving first
to do (something) before (something or someone else); to be ahead of (others, the times, etc.)
previous generation (of a family); previous head of the family; one's (late) father
moving first; person with the first move
先 is thought to combine 之 (footprint, advance) with 儿 (person), suggesting a person stepping forward or going ahead. The modern form retains the idea of precedence.
The top part looks like a foot stepping forward, and the bottom 儿 is a person. Picture someone taking the first step ahead of others: 先 means before or ahead.
For セン, imagine a centipede racing ahead, its many legs always one step before the rest: cent -> セン, and it's always in front.