Telling

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Old English tellan ‘relate, count, estimate’, of Germanic origin; related to German zählen ‘reckon, count’, erzählen ‘recount, relate’, also to tale.

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telling

Gerund from the verb tell, from tell +‎ -ing.

etymonline

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telling (adj.)

"having effect or force," 1852, past-participle adjective from tell (v.).