たいそう・大層・大そうtaisou[1]atamadaka (head-high)

1. adverbvery; extremely; exceedingly; greatly; terribly
   これはたいそう快適かいてきな暖炉だんろですね。↗

   This is a very nice fireplace.
2. na-adjectiveextreme; excessive; great many; large number of
3. na-adjectivesplendid; impressive; tremendous; immense
4. na-adjectiveexaggerated

• Synonym誠にまことにFormalFormal; conveys earnest sincerity, typical in solemn apologies and thanks such as 「誠に申し訳ない」.
• SynonymずいぶんNeutralConsiderably, quite, with a shade of the speaker's surprise; 「大層」 can likewise sound exaggerated or ironic.
• SynonymとてもNeutralThe most common everyday choice, neutral in tone; 「大層」 carries an old-fashioned, exaggerated flavor.
• Synonym大変たいへんNeutralCommon in speech; 「大変だ」 by itself also means a situation is serious or dreadful.
• Synonym甚だはなはだWrittenWritten/formal prose; typically before negative assessments, as in 「甚だ遺憾」.
• Synonym非常にひじょうにNeutralLeans formal, more solemn than 「とても」.
• SynonymごくNeutralAn intensifier, typically with words like ordinary or few — 「ごく普通」「ごく少数」; it does not mark a superlative.
• Synonym極めてきわめてWrittenWritten and formal, as in 「極めて重要」.
• SynonymひどくNeutralTypically modifies negative states, as in 「ひどく疲れた」.
• Synonym実にじつにFormalSame word as 「誠に」 (the first entry); formal, for solemn thanks and apologies such as 「誠に申し訳ない」.
• Synonym大いにおおいにNeutralGreatly, to the full; typically modifies actions, as in 「大いに楽しむ」「大いに反省する」.
• SynonymよほどNeutralQuite, considerably; also 'to a large extent', as in 「よほど疲れたのだろう」.
• SynonymよっぽどCasualColloquial, emphatic form of 「余程」, as in 「よっぽど好きなんだ」.
• Synonym至っていたってWrittenOften in 「至って元気」; calm, measured tone.
• SynonymだいぶNeutralConsiderable degree, especially of change, as in 「だいぶ良くなった」.
• SynonymいかにもNeutralStresses how well something fits the expected image, as in 「いかにも彼らしい」.
• SynonymとびきりCasualColloquial; exceptionally good, a cut above, as in 「とびきりの笑顔」.
• SynonymなかなかNeutralQuite, often more than expected, as in 「なかなか上手」.
• Synonym何ともなんともNeutralDefies description, or, before a negative, 'not at all' — as in 「何とも言えない」.
• Synonym至極しごくWrittenWritten prose with a solemn ring, as in 「至極当然」.
• SynonymすこぶるWrittenWritten prose with an old-fashioned flavor, as in 「すこぶる元気」.
• SynonymめっぽうCasualColloquial; extraordinarily, as in 「めっぽう強い」, with an Edo-era flavor.
• Related最ももっともNeutralUsed in comparisons to mark the superlative, as in 「最も高い山」.
• RelatedめっきりNeutralMarks a clearly visible change, as in 「めっきり寒くなる」.
• Antonym粗末そまつNeutralCrude and shabby; the opposite of 「大層」 in its grand, showy use.
• Antonym些細ささいNeutralTrivial, insignificant; the opposite of 「大層」 in its exaggerating use.

Use 「大層」 for a very high degree or an exaggerated, grand effect, as in 「大層喜ぶ」 and 「大層な口をきく」. In everyday speech say 「とても」 or 「大変」; in formal contexts use 「非常に」 or 「極めて」; for solemn thanks and apologies choose 「誠に」. For shabby, crude things use 「粗末」, and for trivial matters 「些細」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 程度ていどが甚はなはだしいさま。
2. 立派りっぱであるさま。大掛おおがかりであるさま。
3. 大袈裟おおげさなさま。仰々ぎょうぎょうしいさま。
4. とても。非常ひじょうに。