To make soft sniffing sounds through the nose, usually when crying, sneezing, or having a cold.
From 'sniff' (imitative of the sound) plus '-le' (frequentative suffix indicating repeated action), appearing in English by the 1700s.
The '-le' ending is genius for onomatopoeia—it turns a single 'sniff' into that pathetic, repeated 'sniffle-sniffle' that perfectly mimics a sick, crying, or cold-afflicted person.
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