thread, line
綫 centers on a thin, elongated strand or mark: a thread or a line. Both meanings share the idea of something fine and continuous.
綫 combines 糸 (thread) with 戔, which likely contributes the sound and may relate to smallness or thinness. The exact historical development of 戔 is uncertain, but the phono-semantic structure is clear.
The left side 糸 is thread, and the right side 戔 looks like two small spears 戈 stacked. Imagine a thread so fine it can be drawn between the tips of two tiny spears, forming a thin line.
For セン, picture a thin thread being sent through the eye of a needle: send -> セン, and the thread becomes a line.