牲 (shēng) is a bound form (b.f.), meaning it cannot be used alone as an independent word. It always appears as part of a compound, most commonly in 牲口 (shēngkou, 'livestock'), 牲畜 (shēngchù, 'domestic animals'), and 牺牲 (xīshēng, 'sacrifice'). The sense of 'sacrifice' comes from the whole word 牺牲, not from 牲 alone. When you see 牲 in a text, it is always attached to another character.