regardless

From regard +‎ -less.

• (General American) IPA^(key): /ɹɪˈɡɑɹd.lɪs/
• (Received Pronunciation) (BE): IPA^(key): /ɹɪˈɡɑːd.lɪs/

• Audio (Southern England):

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regardless (comparative more regardless, superlative most regardless)

1. Without attention to warnings or indications of bad consequences.

   Synonyms: irregardless, irrespective, anyhow, anyway, at any rate, come what may, in any case, Thesaurus:regardless

   Tatiana knew the meat had passed its expiry date, but she ate it regardless.

regardless (comparative more regardless, superlative most regardless)

1. Paying no attention to; ignoring.
   • 1764 December 24 (indicated as 1765), Onuphrio Muralto, translated by William Marshal [pseudonyms; Horace Walpole], chapter V, in The Castle of Otranto, […], London: […] Tho[mas] Lownds […], →OCLC, pages 190–191:

     Hippolita, ſcarce more alive than her daughter, was regardleſs of every thing but her: but when the tender Iſabella’s care would have likewiſe removed her, while the ſurgeons examined Matilda’s wound, ſhe cried, remove me! never! never! I lived but in her, and will expire with her.

2. Taking insufficient regard of; heedless; careless.

   Synonyms: see Thesaurus:careless

   • 1815 December (indicated as 1816), [Jane Austen], chapter XV, in Emma: […], volume III, London: […] [Charles Roworth and James Moyes] for John Murray, →OCLC, page 279:

     Always deceived in fact by his own wishes, and regardless of little besides his own convenience.

   • 1843, Edgar Allan Poe, The Black Cat:

     I grew, day by day, more moody, more irritable, more regardless of the feelings of others.

regardless

1. (proscribed) Synonym of regardless of; paying no attention to.

   Tatiana ate the cauliflower regardless its colour.

• The phrase 'regardless to' is generally considered incorrect.
• Phrases 'regardless if', 'regardless of if' and 'regardless whether' are generally considered incorrect, with 'regardless of whether' being the preferred wording.