heartstring

noun4

1. •
   (figurative, rare) A source of one's deepest emotions or inner feelings, especially compassion or love.
2. •
   (rare) singular of heartstrings
3. •
   (rare, historical) A tendon or other structure resembling a cord once thought to be attached to, or to brace, the heart.
4. •
   (figurative, no longer used) The essential or vital part of something; the core, the essence, the heart.