Horrendously rhymes with:
tremendously, tremendously, tremendously, aimlessly, ambitiously, anonymously, anxiously, assiduously, autonomously, breathlessly, callously, carelessly, cautiously, ceaselessly, conscientiously, consciously, conspicuously, contemporaneously, contemptuously, continuously, copiously, courageously, curiously, dangerously, disastrously, effortlessly, egregiously, endlessly, enormously, enviously, erroneously, expeditiously, extraneously, fabulously, facetiously, fallaciously, famously, flawlessly, frivolously, fruitlessly, furiously, generously, gloriously, graciously, gratuitously, grievously, harmlessly, harmoniously, hellaciously, helplessly, hideously, hilariously, hopelessly, humorously, incongruously, indigenously, indubiously, ingeniously, instantaneously, intravenously, jealously, laboriously, listlessly, ludicrously, marvelously, mercilessly, meticulously, mindlessly, miraculously, mysteriously, needlessly, nervously, notoriously, obviously, ominously, ostentatiously, outrageously, overzealously, painlessly, perilously, piously, posthumously, precariously, precipitously, previously, purposely, recklessly, relentlessly, religiously, restlessly, ridiculously, righteously, rigorously, ruthlessly, scrupulously, seamlessly, seriously, shamelessly, simultaneously, spontaneously, strenuously, studiously, subconsciously, surreptitiously, suspiciously, synonymously, tediously, tenaciously, tirelessly, tremendously, tremulously, unambiguously, unanimously, unceremoniously, unconsciously, variously, vicariously, viciously, vigorously, vociferously, zealously