استبطأه doesn't appear in Hans Wehr's Modern Standard Arabic dictionary, but Lane's Arabic-English Lexicon has an entry for it. Select the entry below for the full classical Arabic definition.

• استبطأهُ Form 10

  10 استبطأهُ ذ (S, TA) He deemed him, or reckoned him, slow, tardy, &c. (KL.) You say, كَتَبَ إِلَىَّ يَسْتَبْطِئُنِى [He wrote to me, deeming me, or reckoning me, slow, &c.]. (TA.)…

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